Method of preserving colloid films



METHOD OF PRESERVING COLLOID FILMS Filed Sept. 19, 1952 Patented Mar. 5, 1935 UNITED STATES PATENT OFFICE George S. Rowell, Cleveland, Ohio, assigner to Multigraph Company, Wilmington, Del., a corporation of Delaware Application September 19, 1932, Serial No. 633,823

8 Claims.

'This invention relates to the preparation and preservation of the actinic sensitiveness of chromated colloid films, and it comprises coating a freshly dried film of chromated colloid on a suitable support in a non-actinic light with a transparent film of water-insoluble, but preferably water-permeable, waxy material, whereby the actinic sensitiveness and the surface continuity of the chromated colloid film is maintainable.

Light-sensitive chromated colloid films have been extensively used in the graphic arts; and, whether the colloid, sensitized by bichromates or chromates, was glue, gelatin, albumin or other colloid, it was invariably the custom to use the prepared material within a few hours after it was dried. Not only would the actinic sensitiveness diminish with time, but the resistance to swelling of the light-hardened proteid progressively diminished with the time elapsing between drying the sensitive layer and the development thereof. Furthermore, if a film of the chromated colloid were dried to a moisture content much below that at which it first became palpably dry, it exhibited a tendency to shrink and crack. 'I'his tendency to form minute surface cracks upon excessive drying is accentuated in films of uneven thickness, such as inevitably result from fiooding a grained lithographic plate with a thin film of chromated colloid.

As is well recognized in the art, a chromated colloid film must be relatively thin in order that it may be hardened through to the base support by the action of light. The depressions of the grained surface of a lithographie plate therefore produce appreciable variations in the thickness of a film on such surfaces, and in consequence when such a film shrinks and cracks, the cracks are determined by and correspond to the variations in thickness produced by the grain depressions, and hence are distributed over the entire plate surface. Even if the actinic sensitiveness of a dry chromated colloid film were maintained, these minute cracks would render the film unfit for lithographie reproduction.

Lithography depends upon the immiscibility of water and oil and the chromated colloid lighthardened image surfaces serve as a base for the ink which is to be transferred to paper. The non-printing areas of the plate must be maintained free of greasy ink and readily wettable by water and therefore must be entirely freed of the chromated colloid not an essential part of the image. In order to facilitate development of a light-hardened colloid image, it is essential that the operator should be able to visually determine when all the non-hardened colloid has been washed off the non-image areas of the plate. It is also desirable to condition the light-hardened or retained colloid constituting the image for grease receptivity by contact of the same with a so-called developing ink before developing the image with water. If, however, the chromated colloid film has been subjected to conditions conducive to excessive loss of moisture, shrinkage and consequent cracking occurs, as stated, and upon treating the exposed surface with developing ink, this ink passes through these cracks and reaches the surface of the underlying plate. Where these cracks underlie non-printing areas the effect of such contact of the plate by the developing ink is disastrous as each inky delineation of a crack on the plate will ultimately pick up enough printing ink to record itself in the highlights of the printed image.

It is an object of my invention to so prepare and protect a dried chromated colloid film on a planographic printing plate as to permit of the storage of such plate for an indefinite period withoutappreciable loss of actinic sensitivity and without detrimental surface cracking. Other objects and advantages will become apparent from its more detailed description, and from an examination of the accompanying drawing which shows in exaggerated cross section a plate bearing an actinically sensitive film protected in ac.- cordance with my invention.

It is well known in the art that the effect of the` exposure of a chromated colloid film to light depends not only upon the intensity of the light and the duration of the exposure but upon the elapsed time between exposure and development. The effect of a given exposure increases with such elapsed time. It is also well known that a chromated colloid lm becomes light-sensitive only when dry.

I have discovered that if I dry a film of chromated colloid supported on the surface of a planographic printing plate in non-actinic light, that is, a light which produces no measurable hardening effect on the film regardless of the time which may elapse between drying and developing, and after drying until the last visible traces of moisture have disappeared and the film is palpably dry, I coat the film with a thin layer of a water-insoluble transparent waxy material and then seal the so dried and protected film in' a light-tight envelope; the actinic sensitiveness of the film will persist substantially undiminished, if not indefinitely, at least for a year or more, and that upon removal of the waxy film by means of a suitable solvent it will be found that the integrity of the surface continuity has also persisted, i. e., the surface will be found to be free from detrimental cracks.

A suitable water-insoluble waxy nlm may be obtained from a solvent solution of wax, paranin, or the like, but obviously such a film must be removed by the use of a suitable non-aqueous solvent before 4the underlying colloid may be developed with water. 1

I have further discovered as part of this invention, however, that all the advantages of the waxy nlm above described, but without the disadvantages attendant upon the necessity for the removal of this waxy coating by a non-aqueous solvent, may be obtained by the use of a nlm which. though water-insoluble, is water-permeable. Any such water-insoluble nlm, which is however sufficiently water-permeable to permit water to get at and dissolve the underlying unhardened chromated colloid and which is readily carried away with and by the dissolving colloid, i. e., which is not inherently tough, is suitable for my purpose. 'Preferably I use materials which tend to remain nrmly adherent to the light-hardened image area. The following formula is illustrative of compositions suitable for my purpose:

Ounces Beeswax s. il/g Tallow 3 I have also discovered that I may avoid the use of organic solvents by emulsifying wax alone or preferably wax and tallow in water, coating the dried colloid nlm with such emulsion and by subsequently removing the aqueous phase by desiccation.

Not only should the drying of the chromated colloid nlm be carried out in the dark, or at least in a non-actinic light, but the subsequent coating operation should also be so carried out as at all times to avoid subjecting the nlm to any light capable of exerting a hardening action on the colloid. I find that the usual whirling apparatus is a satisfactory means of obtaining both the primary light-sensitive coating and the superimposed protective waxy coating. In general I nnd it desirable to apply as thin a nlm as will yield an unbroken continuous surface, except in the case of the emulsion where a nlm of the order of two or three tenthousandths of an inch is desirable. The protected light-sensitive nlm of this invention may. after protracted storage. be exposed and developed in the usual way except when the protective coating is water impermeable, in which strictly chemical waxes, which have in common the physical characteristic of unctuousness.

I claim:

1. A method of preserving actinically sensitive chromated colloid nlms, comprising as steps treating the freshly dried nlm with a coating of water insoluble but water permeable waxy material and drying the same.

2. A method of preserving actinically 'sensi tive chromated colloid nlms, comprising as steps treating the freshy=dried nlm with an emulsion coating of water insoluble but water-permeable waxy material and drying the same.

3. A method of preparing freshly dried light sensitive chromated colloid nlms, on planographic printing surfaces, comprising as a'step preserving the surface with a chromated colloid, drying, coating with a water insoluble waxy material, and, when ready for use, exposing the nlm through said coating and developing the required image in said nlm, the coating remaining adherent to the light-hardened image area.

4. A method of preparing freshly dried light sensitive chromated colloid nlms, on planographic printing surfaces, comprising as a step preserving the surface with a chromated colloid, drying,V

coating with a water insoluble but water permeable waxy material, and, when ready for use, exposing the nlm through the coating and developing the required image in said nlm.

5. Actinically sensitive chromated colloid nlms, characterized by being palpably dry and by having a coating at water insoluble waxy material.

6. Actinically sensitive chromated colloid nlms, characterized by being palpably dry and by having a coating of water insoluble but water permeable waxy material.

7. A planographic printing surface having a light sensitive nlm of chromated colloid and a coating of water insoluble waxy material.

8. A planographic printing surface having a light sensitive nlm of chromated colloid and a coating of water insoluble but waterv permeable waxy material. q

- GEORGE S. ROWELL. 

